Graph data, such as chemical networks and social networks, may be deemed
confidential/private because the data owner often spends lots of resources
collecting the data or the data contains sensitive information, e.g., social
relationships. Recently, neural networks were extended to graph data, which are
known as graph neural networks (GNNs). Due to their superior performance, GNNs
have many applications, such as healthcare analytics, recommender systems, and
fraud detection. In this work, we propose the first attacks to steal a graph
from the outputs of a GNN model that is trained on the graph. Specifically,
given a black-box access to a GNN model, our attacks can infer whether there
exists a link between any pair of nodes in the graph used to train the model.
We call our attacks link stealing attacks. We propose a threat model to
systematically characterize an adversary's background knowledge along three
dimensions which in total leads to a comprehensive taxonomy of 8 different link
stealing attacks. We propose multiple novel methods to realize these 8 attacks.
Extensive experiments on 8 real-world datasets show that our attacks are
effective at stealing links, e.g., AUC (area under the ROC curve) is above 0.95
in multiple cases. Our results indicate that the outputs of a GNN model reveal
rich information about the structure of the graph used to train the model.Comment: To appear in the 30th Usenix Security Symposium, August 2021,
Vancouver, B.C., Canad